Cryptosporidium is an apicomplexan parasite and one of the most important causes of severe diarrheal disease. More than 50% of U.S. outbreaks linked to water use are caused by Cryptosporidium and the parasite is thus considered a category B potential bioterrorism agent. Immunosuppressed patients are extremely susceptible and suffer life-threatening chronic disease, cryptosporidiosis is an original AIDS defining opportunistic infections. Globally, children under the age of two bear the main disease burden and Cryptosporidium is the second most important diarrheal pathogen in this group. Over the initial period of support we developed stable transformation, a series of reporter genes and CRISPR-Cas9 mediated gene knock out. We apply and further hone these new tools to investigate how this intracellular parasite manipulates its host cell. Cryptosporidium thrives in a uniquely remodeled cellular niche cells that is critical to pathogenesis, drug susceptibility and immune interaction. Our central hypothesis is that the parasite injects proteins directly into host enterocytes and that those proteins are the mechanistic effectors of invasion, remodeling and immune evasion. We have discovered the first set of such factors in C. parvum. This application will use an array of modern approaches to define the parasite's exported proteome and to understand the function of effector proteins to first establish and then maintain parasite infection.